Summary:
Adds the initial extension that sets up the ctreemanifest. It currently relies
on the fastmanifest extension to hook into all the manifest APIs to construct
ctreemanifests.
Test Plan:
In a future patch, I was able to run 'hg manifests' on a commit and
have it return the manifest contents by reading the treemanifest.
Reviewers: #fastmanifest, ttung
Reviewed By: ttung
Subscribers: ttung
Differential Revision: https://phabricator.intern.facebook.com/D3755327
Signature: t1:3755327:1472114482:0c5862cba68ed4db643d28c2fae01f33f5352970